Effect of acute hyperthyroidism on insulin removal in the rats

The aim of this work was to study secretion and removal of insulin in hyperthyroidism. The experiments were carried out on two groups of male Wistar rats: control and hyperthyroid. Acute hyperthyyreosis was induced by administration of L-thyroxine (Fluka A.G.) 0.8 mg/kg for 21 days. Each group was divided into three subgroups: fed, fasted 24 h, fasted 24 h-treated with glucose. The insulin concentration was determined in the portal and aortal blood and a difference between the two concentrations was considered as reflecting insulin removal. The plasma insulin concentrations in both vessels of fed as well fasted hyperthyroid group were higher from the respective values in the control one. The removal of the hormone in the fed rats was similar in the both groups. Fasting reduced insulin removal only in the control group. Treatment of the fasted rats with glucose abolishes this difference. The liver glycogen content was similar in the two fasted groups but the blood glucose level was higher in the hyperthyroid than in the control rats. The latter factor could account for the difference in insulin removal between the two groups. It is concluded that hyperthyroidism increases insulin secretion but does not affect its removal.